1. Field of the Invention
The present invention relates to an ultrasonic diagnostic method and apparatus, and more particularly to such a method and apparatus which is capable of producing a sectional image of a living body by projecting an ultrasonic beam onto the living body and receiving an echo reflected from the interface between heterogeneous constituents whose acoustic impedances are different from each other.
2. Description of the Prior Art
In conventional equipment of the type known heretofore, the scanning of an ultrasonic beam is performed by either a linear scanning system or a sector scanning system.
The linear scanning is carried out as illustrated in FIG. 1 by the use of a vibrator array 10 consisting of a plurality of vibrators, wherein the vibrators are driven sequentially in such a manner that some (e.g. three) of them grouped as a.sub.1 -a.sub.2 -a.sub.3, a.sub.2 -a.sub.3 -a.sub.4, etc. are excited in such groups to cause a sequential parallel shift of an ultrasonic beam which is substantially perpendicular to the vibrator array 10. The reason for grouping three vibrators for each excitation is to improve the directivity of the ultrasonic beam.
Sector scanning is carried out as illustrated in FIG. 2, wherein the entire array of vibrators 10 are driven at individually different timings such that the ultrasonic beam as a whole is turned to a certain direction, which is then changed sequentially to deflect the ultrasonic beam in the shape of a sector.
The linear scanning system is disadvantageous in that the range (visual field) of the image obtained is limited to the width L of the vibrator array 10. Consequently, in order to widen the visual field, the number of vibrators must be increased to widen the array 10 increasing the complexity of the system. Moreover, another disadvantage is the deteriorating degree of intimate contact with the wall of the living body. As for the sector scanning system, it also has some disadvantages including a narrow visual field at a short distance and an unsatisfactory resolution for long-distance images.